Electric drives, such as AC motors used in vehicle applications, are typically controlled via a voltage source inverter system. Discontinuous Pulse Width Modulation (DPWM) methods are commonly employed to control the fundamental output voltage component of three phase voltage source inverters. These three-phase voltage source inverters may in turn be used to control the phase currents of three phase AC motors. DPWM methods typically reduce inverter losses in comparison with continuous Pulse Width Modulation (PWM) methods, such as sinusoidal or space vector modulation.
DPWM methods generally differ from continuous PWM methods in that DPWM methods use a single zero vector in a given switching cycle of the three phase voltage source inverter. Additionally, in most DPWM methods, each switch of the three phase voltage source inverter is closed, or clamped, for sixty-degree (60°) segments of a switching cycle. The location of the sixty-degree (60°) clamped segment, with respect to the output voltage of the three phase voltage source inverter and the load power factor, generally determines the type of DPWM method and resulting PWM properties.
PWM techniques typically contribute a ripple current to the motor currents. In conventional voltage source inverters, the actual applied output voltage of the voltage source inverter includes an AC component resulting from the PWM action. This AC component is composed of harmonic voltages corresponding to each state of the PWM pulse sequence and may be a source of errors in the sampling instant of the motor drive. For example, inverter dead-time is typically selected to account for variations in temperature and component tolerances. As a result, the actual switching instant for the voltage source inverter is not necessarily constant and may shift in time and manifest as if the sampling instant is in error (e.g., a measurement error).
Accordingly, it is desirable to provide a method and system for controlling an electric drive that reduces current sampling errors. Additionally, it is desirable to provide a method and system for controlling an electric drive to improve torque accuracy. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background.